I recently created an account at yandex.mail. When I sent an e-mail to a gmail address that is checked from an iPhone, it was reported that the domain yandex.com is often used by spammers. Do emails from yandex often end up in the spam folder and is it possible to do something about it?

Yandex is commonly used by spammers to the point that it has been rendered unusable as a legitimate email service. As for the filtering, that is done in the receiving end, in this case Gmail, and there’s really nothing you can do about it. Once the sending domain becomes known as a spammers haven, it usually ends up on a blacklist and this is the result.

To test Yandex, I registered a (free) .ml domain, added the domain to my Yandex/Mail acct, and created a functioning mailbox under this domain (=this email now hosted by Yandex). I then tried to add this .ml email as a secondary email on a FB account and FB rejected it as "Invalid email".

To double check that the rejection is not due to the .ml domain, I then registered another .ml domain (good thing .ml domains are free as I don't have unused regular tlds lying around for testing), put it on one of my G Apps/G Suite accounts as an alias domain, and created a functioning mailbox under this domain (=this email now hosted by G Apps). When I tried to add this .ml email as a secondary email on the same FB acct, it was accepted.

So my conclusion is: FB does seem to blacklist emails coming from domains hosted by Yandex (while it doesn't seem to do a blanket blacklist of all .ml domains).
Of course this could be just a coincidence (and FB practice might not be representative of general practice) but at this stage I am not ready to move my domain's MX from G Apps to Yandex Mail...

It seems possible that FB in reaction to the Russian fake news epidemic has put a much stricter filter on account registration obviously from Russia. I think it is unlikely that many services would put a blanket block on anything from Gmail or G Suite or else they would be blocking 75% of the incoming emails.

I never thought that FB would block G Apps/G Suite. I only wanted to make sure that the FB rejection of my first Yandex-hosted .ml email was not due to the fact that FB has decided to block all .ml emails (regardless of MX). That is not the case, obviously.